The invention relates to the field of optical imaging and surveillance. More particularly, the present invention relates to the use of laser illumination of surveillance tags for tracking a tagged moving vehicle.
Covert tracking of a suspect vehicle through traffic is difficult for law enforcement personnel. No known low-cost optical technological aids are currently available to assist with this task. Currently, vehicles are optically tracked using telescoping cameras and telescopes without any visual enhancement. The tracking is usually done by following ground based police cars with overhead helicopters. Often it is difficult to track and follow the suspect vehicle through congested traffic. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a system for covertly tracking a vehicle through traffic.
Another object of the invention is to provide a system for illuminating a xc2xc waveplate tag on a vehicle with pulsating polarized light for optically detecting the tag disposed on a vehicle.
Yet another object of the invention is to provide a system for illuminating a xc2xc waveplate tag with pulsating polarized light for optically detecting the tag disposed on a vehicle for providing an optical reference for following and tracking the vehicle when in transit.
The invention is a system for optically enhancing an image of a tag disposed on a moving vehicle for improved ease of following and tracking the target that may be, for example, a moving vehicle or transitory cargo container. The tag may be a retroreflecting tag or xc2xc waveplate tag on reflecting surface of the target. The tags provide enhanced optical signals recognition and tracking when the target is transitory. The optical enhancement causes the tagged vehicle to stand out prominently from other vehicles thereby significantly improving an ability to track the moving target. In a preferred form, infrared illumination that is not detectable by the human eye is used to optically enhance an image of the tag affixed to the target. The illumination is turned alternately on and off so that the reflected image from the tag pulsates, as observed through an optical device. The system includes an illumination means, viewing means and a tag disposed on the target. The illumination means is preferably a pulsed laser and the monitoring means is preferably a camera, both of which are co-located at a stand-alone tracking unit. Because the tracking unit can be made from solid state devices, the covert tracking unit for carrying out the invention is a rugged, compact, inexpensive device that is readily amenable to wide-spread field use.
The system preferably includes a video camera or camcorder, or other optical imaging means that records during covert tracking. In one preferred form of carrying out the invention, two near infrared laser diodes with internal beam collimating optics are used to illuminate one or more tags for enhanced recognition of the target from background images. The xc2xc waveplate tags consist of a transparent birefringent material that is a quarter waveplate with adhesive backing so that the tags can be readily applied to a reflecting surface of the vehicle being tracked. The xc2xc waveplate tags provide for polarization rotation so that polarized filters associated with the viewing means can isolate images reflected from the tags while blocking other background images. The tags may be disposed, for example, on the rear license plate of the vehicle to be tracked with the license plates acting as retroreflectors. The tags are small and transparent, for example 1-2xe2x80x3 transparent squares, which are unnoticeable to the human eye under casual inspection leading to covert tracking use. In one embodiment, the tags may be simple retroreflectors affixed to a non-reflecting surface of the target. After applying one or more of the tags to the target, the operator of the monitoring camera attempts to keep the target vehicle in the viewfinder of the video camera.
In a preferred form, the monitoring camera can be any commercial camcorder capable of detecting near infrared light at 800-950 nm. The camera is fitted with a polarizer that preferentially transmits linearly polarized light oriented in a given direction, for example, blocking horizontally polarized light and passing vertically polarized light. One of the two laser diode light sources mounted to the camera emits vertically polarized near infrared light at 800-950 nm that is reflected from the retro-reflecting tags on the license plate of the target vehicle. The vertically polarized light is detected by the camera. The quarter waveplate tags that are oriented with their fast and slow axes at 45 degrees to the vertical, rotate the vertically-polarized light from the laser into horizontally-polarized light, and the vertical polarizer on the camera blocks the light reflected from the tags. Thus, the observer sees a bright license plate with dark spots where the tags are placed. The first laser is then turned off and a second laser is turned on. The second laser is identical to the first except that the second laser emits horizontally polarized light. The horizontally polarized light is blocked from detection by the camera for all parts of the scene field of view except the tagged areas that have rotated the horizontal light into vertically polarized light. The result is that by alternately flashing these two lasers at a nominal rate of several hertz that is perceptible to the human eye, the tagged target license plate appears in the camera as a checkerboard whose light and dark areas reverse each time one laser is switched to the other. This dual illumination approach exploits one of the most sensitive of human visual detection capabilities, that of detecting quick changes in a pulsating scene. By focusing the lasers to illuminate predominantly the license plate region of the target vehicle, unpolarized ambient light provides a nearly constant overall background image. Only the illuminated target plate will flicker from dark on bright to bright on dark, dramatically enhancing detection of the tag and the concomitant ability to track the tagged moving vehicle. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.